Troubleshooting IIS

Understanding HTTP Error Messages

HTTP error messages are related to the HTTP status codes which are included in the response headers when pages are requested from the Web server. HTTP status codes are three digit numbers.

HTTP status codes are categorized into the following categories:

Understanding HTTP error messages and their associated status codes is important when you need to troubleshoot IIS. A few common errors on the client side (4xx error) and server side (5xx error) are listed below:

From the above, you can see that a few of the error codes are quite vague in describing the actual issue. For this reason, a few HTTP error messages have substatus codes to provide more information on the current errors. The substatus codes associated with the various HTTP error messages in IIS 6 are:

By default, IIS returns the following default HTTP errors:

Custom HTTP Error Messages

Besides for the default HTTP error messages, IIS returns custom error messages. IIS returns custom error messages for all the other HTTP error messages, other than the default HTTP error messages listed previously. Custom error messages provide more information on the error condition than what the default messages provide, and can be customized. The custom error messages also include additional troubleshooting tips, which the default HTTP error messages do not provide. When custom error messages are enabled, IIS logs the error in the web logs for the site at the same time that it sends the HTTP error message to the client.

You can however configure IIS to return a default message over a custom error message for a particular error, or you can configure IIS to return only default error messages for all errors.

The details and troubleshooting tips associated with custom HTTP error messages are discussed and condensed in the following section:

How to configure IIS to return a default HTTP error message over a custom error message

  1. Open IIS Manager.
  2. Right-click the Web Sites node, and select Properties from the shortcut menu.
  3. Click the Custom Errors tab.
  4. Select an error message from the Error Messages For HTTP Errors list box, and click the Set To Default button.
  5. To return only default HTTP error messages and not custom HTTP error messages, select all the error messages in the Error Messages For HTTP Errors list box, and then click the Set To Default button.
  6. Click OK.

How to customize error messages

  1. Open IIS Manager.
  2. Right-click the Web Sites node, and select Properties from the shortcut menu.
  3. Click the Custom Errors tab.
  4. The HTTP error messages returned by IIS are listed in the Error Messages For HTTP Errors list box.
  5. The majority of these messages are stored in the \Windows\Help\IISHelp\Common directory, and are put into operation as HTML pages.
  6. To customize these messages, use Notepad or Microsoft FrontPage.

How to redirect a user to another Web server or site when an error condition occurs

  1. Open IIS Manager.
  2. Right-click the Web Sites node, and select Properties from the shortcut menu.
  3. Click the Custom Errors tab.
  4. Select the error message from the Error Messages For HTTP Errors list box, and click the Edit button.
  5. The Edit Custom Error Properties for the particular error message opens. The options which you can select in the Message Type list box are:
    • Default, the standard error message is utilized.
    • File, the custom error message is utilized.
    • URL, redirection occurs to the URL where the error message is located.
  6. Click OK.

How to configure worker process recycling

With IIS 6, ASP session information is lost when a worker process stops, or when IIS recycles the worker processes. By default, IIS recycles worker processes every 120 minutes. If you need to maintain session information for a longer period, you can completely disable worker process recycling, or you can change the time interval when worker process recycling occurs.

To configure worker process recycling:

  1. Open IIS Manager.
  2. In the console tree, expand the Application Pools node.
  3. Right-click the appropriate application pool, and select Properties from the shortcut menu to open the Properties dialog box for application pool.
  4. Click the Recycling tab.
  5. To extend the time period for when the worker processes are recycled, change the value in the available box.
  6. To completely disable worker process recycling, clear the Recycle Worker Process (In Minutes) checkbox.
  7. Click OK.

How to configure the queue length of an application pool

Connection errors are usually caused by the request queue length exceeding the quantity of available application pools, or by rapid-fail protection being started by IIS. The default queue length value is 2,000 requests.

To change the default queue length:

  1. Open the IIS Manager.
  2. In the console tree, expand the Application Pools node.
  3. Right-click the appropriate application pool, and select Properties from the shortcut menu to open the Properties dialog box for application pool.
  4. Click the Performance tab.
  5. Select the Limit the kernel request queue checkbox, and enter the value in the available box.
  6. Click OK.

To configure rapid-fail protection:

  1. Open the IIS Manager.
  2. In the console tree, expand the Application Pools node.
  3. Right-click the appropriate application pool, and select Properties from the shortcut menu to open the Properties dialog box for application pool.
  4. Click the Health tab.
  5. Enter a value for Failures, and a value for Time Period (In Minutes) in the Enable Rapid-Fail Protection area.
  6. Click OK.


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